Recently, radio broadcast is digitalized in response to digitalization of TV broadcast. For example, digitalized terrestrial radio broadcast and digitalized satellite radio broadcast start to provide service. The digitalized satellite radio broadcast is charged. However, terrestrial HD (i.e., high definition) radio broadcast becomes common in US, and it is not charged. Here, the terrestrial HD radio broadcast has commercial messages. Thus, expansion of market in the digitalized radio broadcast is expected.
The HD radio broadcast uses an in-band on-channel (i.e., IBOC) system for adding an electric wave of a digital signal to another electric wave of an analog signal. Specifically, the digitalized radio broadcast is added to conventional analog radio broadcast. Thus, an analog radio receiver can receive the analog radio broadcast, and further, a HD radio receiver can receive the digitalized radio broadcast. When the digitalized AM radio broadcast is received by the receiver, FM-quality sound is obtained. When the digitalized FM radio broadcast is received by the receiver, CD-quality sound is obtained.
FIG. 4 shows an example of spectrum of a FM radio wave in an IBOC system according to a related art. A pair of side bands is disposed on both sides of a carrier wave of the analog FM broadcast. Each side band occupies spectrum in a range between f1−198 kHz and f1−129 kHz or a range between f1+129 kHz and f1+198 kHz. Here, f1 represents a center frequency. The side band includes a sub-carrier wave generated by an orthogonal frequency division multiplex (i.e., OFDM) modulation method. Thus, the digital FM broadcast is performed. A signal level of the OFDM sub-carrier wave corresponding to the digital FM broadcast is set to be −25 dBμV with reference to a carrier wave corresponding to the conventional analog FM broadcast.
The radio receiver has automatic tuning function for searching or tuning a radio broadcast station automatically so that an audio signal from the station is clearly received. The FM radio station has a frequency clearance of, for example, 200 kHz, in US. Accordingly, a frequency step of the radio receiver is set to be 200 kHz when the radio receiver tunes the station.
However, the digital FM radio wave has a wide frequency range of 400 kHz, for example, as shown in FIG. 4. Thus, the analog radio receiver may fail to tune the station at a frequency next to the central frequency of the station by one step, i.e., by 200 kHz. Thus, the analog radio receiver may receive only noise even when tuning of the station is performed. Here, the analog radio receiver cannot demodulate and decode the digital signal.
JP-A-2004-312563 and JP-A-2005-191850 disclose a technique for automatically tuning a station when the analog broadcast and the digital broadcast are mixed. An automatic tuning device disclosed in JP-A-2004-312563 includes a means for canceling frequency information of a station, which is determined by the device that the device cannot output a normal audio signal, the frequency information detected by the receiver. Determination whether the device can output the normal audio signal is performed on the basis of current position information of the receiver and information of the station memorized in a memory. The information of the station is, for example, a channel of the station, an output power of the station, and a latitude and a longitude of the station. In this case, it is necessary to preliminarily memorize the information of the station in the memory, the station which is not preset by the device. Accordingly, it is necessary for the device to update the information when a new station increases in a region. Specifically, in a region at which the digital broadcast rapidly becomes common, the update of the information is necessary.
Further, the radio receiver disclosed in JP-A-2005-191850 searches the station with distinguishing the analog radio broadcast and the digital radio broadcast so that the receiver receives only the digital signal in the digital radio broadcast. In this case, the radio receiver receives both of the analog radio broadcast and the digital radio broadcast. Accordingly, it is impossible to apply the above technique to a conventional analog receiver having no detection circuit for the digital radio broadcast.
Thus, it is required for a radio broadcast receiver having no detection circuit for a digital radio broadcast signal to tune only an analog radio broadcast station with high accuracy. Further, it is required for an automatic tuning device for an analog radio broadcast station with high accuracy.